I. Field of the Invention
The present invention relates generally to magnetic spin resonance methods, more specifically nuclear magnetic resonance (NMR) methods. It is concerned with the acquisition of data for the formation of two and three dimensional images of the spin density distribution in a material containing electron or nuclear spins.
II. Description of the Prior Art
NMR imaging is a relatively new technique for obtaining cross-sectional pictures through the human body (and other objects) without exposing the body to ionizing radiation, see e.g. the article "NMR Imaging in Medicine" by Ian L. Pykett in SCIENTIFIC AMERICAN, May 1982, pp. 54 to 64.
In U.S. patent application No. 828,956 to Frahm et al, now U.S. Pat. No. 4,707,658, incorporated herein by reference thereto, there is described a NMR method which has become known as fast low angle shot (FLASH) method. In typical embodiments of the FLASH method, only a fraction of the spins which are aligned in a Z direction by a constant magnetic field, are tilted or "flipped" by a space selective radio frequency (rf) pulse having a flip angle less than 90 degrees into an XY plane and the flipped spins are utilized as completely as possible to generate a gradient echo signal by gradient switching. Typically, flip angles between about 5 and 60 degrees are used which yield an optimum spin density or T.sub.1 contrast. Repetition rates with TR periods down to only 20 milliseconds and less are possible.
The FLASH method utilizes an at least approximate equilibrium state of the longitudinal or Z magnetization which is established by the train of "fractional rf pulses" having the specified small flip angles. The transverse magnetization in the XY plane can be neglected if the spin-spin relaxation time T.sub.2 is in the order of e. g. 100 milliseconds and less which applies to most tissues in vivo. However, some tissues and body fluids, as the cerebrospinal liquid, exhibit T.sub.2 values of 1 second and longer. In such cases some transverse magnetization remains at the end of the TR period of each partial experiment and this remaining transverse magnetization is transformed by the next rf pulse into an undesirable phase coherence so that it manifests itself as a spurious spin echo or stimulated echo. These SSFP type signals (SSFP=Steady State Free Procession) cause artifacts in rapid NMR images of the brain and the like. It has been proposed in an article "Phase and Intensity Anomalies in Fourier Transform NMR" by Freeman et al. in JOURNAL OF MAGNETIC RESONANCE 4, 366-383 (1971) in connection with a method of obtaining Fourier transform spectra (i. e. a non-imaging NMR method), to destroy spurious transverse magnetizations occurring after repeated rf excitation, by field gradient pulses which have a randomly varying amplitude or duration and prevent refocussing. This known device cannot be used in combination with the FLASH method, since gradient pulses having randomly varying amplitudes would cause inacceptable image distortions which are believed to be produced by induced eddy currents.
A modification of the FLASH imaging method, known as the FISP method, in which the spurious transverse magnetization remaining at the end of a partial experiment is refocussed by repeating, in reverse order, all of the gradient pulses is disclosed in a publication by Oppelt et al. "FISP: Eine neue schnelle Pulssequenz fur die Kernspin-Tomographie" (FISP: A new fast pulse sequence for NMR imaging) in Electromedica 54, 16-18 (1986). However, this increases the measuring time and eliminates T.sub.1 contrast capabilities.